Undercooling and rapid solidification of Nb-Si eutectic alloys studied by long drop tube
(1.北京市科学院力学研究所国家微重力实验室2.National Microgravity Laboratory, Institute of Mechanics,Chinese Academy of Sciences, Beijing 100080, China)
Abstract: Niobium-silicide alloys have great potential for high temperature turbine applications. The two-phase Nb/Nb5Si3 in situ composites exhibit a good balance in mechanical properties. Using the 52 m drop tube, the effect of undercooling and rapid solidification on the solidification process and micro-structural characterization of Nb-Si eutectic alloy was studied. The microstructures of the Nb-Si composites were investigated by optics microscope (OM), X-ray diffraction (XRD) and scanning electron microscope (SEM) equipped with X-ray energy dispersive spectrometry (EDS). Up to 480 K, deep undercooling of the Nb-Si eutectic samples was successfully obtained, which corresponds to 25% of the liquidus temperature. Contrasting to the conventional microstructure usually found in the Nb-Si eutectic alloy, the microstructure of the undercooled sample is divided into the fine and coarse regions. The most commonly observed microstructure is Nb+Nb5Si3, and the Nb3Si phase is not be found. The change of coarseness of microstructure is due to different cooling rates during and after recalescence. The large undercooling is sufficient to completely bypass the high temperature phase field.
Key words: Nd-Si eutectic alloys; intermetallics; long drop tube; undercooling; rapid solidification